energies Review Review of Voltage and Frequency Grid Code Specifications for Electrical Energy Storage Applications Xing Luo 1,*, Jihong Wang 1,* ID , Jacek D. Wojcik 1, Jianguo Wang 1, Decai Li 1, Mihai Draganescu 2, Yaowang Li 3 and Shihong Miao 3 1 School of Engineering, University of Warwick, Coventry CV4 7AL, UK; [email protected] (J.D.W.); [email protected] (J.W.); [email protected] (D.L.) 2 UK National Grid, Warwick CV34 6DA, UK; [email protected] 3 School of Electrical & Electronic Engineering, Huazhong University of Science & Technology, Wuhan 430074, China; [email protected] (Y.L.); [email protected] (S.M.) * Correspondence: [email protected] (X.L.); [email protected] (J.W.); Tel.: +44-024-765-23780 (J.W.); Fax: +44-024-764-18922 (J.W.) Received: 12 April 2018; Accepted: 24 April 2018; Published: 26 April 2018 Abstract: To ensure the stability and reliability of the power network operation, a number of Grid Codes have been used to specify the technical boundary requirements for different countries and areas. With the fast propagation of the usage of Electrical Energy Storage (EES), it is quite important to study how the EES technology with its development can help the Grid Code realization. The paper provides a comprehensive study of Great Britain (GB) Grid Code mainly on its voltage and frequency relevant specifications, with a comparison of other countries’ grid operation regulations. The different types of EES technologies with their technical characteristics in relation to meeting Grid Codes have been analysed. From the study, apart from direct grid-connection to provide grid services on meeting Grid Codes, EES devices with different technologies can be used as auxiliary units in fossil-fuelled power plants and renewable generation to support the whole systems’ operation. The paper also evaluates the potentials of different types of EES technologies for implementing the relevant applications based on the Grid Codes. Some recommendations are given at the end, for the EES technology development to help the Grid Code realization and to support the relevant applications. Keywords: electrical power system; grid code; electrical energy storage; electricity generation; frequency response and control; low voltage ride through; grid-connection 1. Introduction A power network can be a quite complex system which is from electricity generation, transmission, and distribution to end-user consumption. To ensure the stability and reliability of such a system operation, a series of specifications entitled Grid Code normally issued by Transmission System Operators (TSOs) have been set and implemented to specify the technical boundary requirements relating to connections to, and the operation and use of, the electricity network [1]. The Grid Code involves many aspects of the power grid operation and thus its contents have a wide range. Electrical Energy Storage (EES) has been recognized as an important part of power networks in recent years because it can have multiple attractive functions to power networks, e.g., reducing CO2 and other greenhouse gas emissions, supporting meeting peak load demands, improving the electrical power quality and helping in the smart grid realization [2–5]. With the different EES technologies, EES systems can be used either as auxiliary facilities in power plants (including fossil-fuelled and renewable power generation) or as independent units in the power networks to support the Grid Code realization. Energies 2018, 11, 1070; doi:10.3390/en11051070 www.mdpi.com/journal/energies Energies 2018, 11, 1070 2 of 26 Although in many countries the Grid Code has not been updated yet with EES specific prescriptions, the EES systems can be treated as electricity generation utilities in the grids when they are operated in the discharging mode with grid-connection. Thus, the examination of EES technical characteristics in relation to the current power network operation regulations is essential for the improvement of power network stability, the guidance of EES technology development and the Gide Code evolution with the fast propagation of renewable in power networks. The study on the different EES technologies with the purpose of their implementations in compliance with the power network regulations, supporting the Grid Code realization and impacting its evolution is relatively lacking. So far, there are some good quality papers which have mainly focused on the review of the research and development of EES technologies with their power system applications. Amirante et al. provided a detailed overview of the state-of-the-art EES technologies, covering mechanical, electrochemical and hydrogen technologies [4]. The operation principles, technical and economic features of different EES options were analysed, and a schematic comparison among the potential utilization of EES systems was presented [4]. Robyns et al. [3] highlighted the challenge and the valuation of EES in transportation systems with concerned electrical power systems, e.g., local grids for applications in aviation, electrical vehicles, hybrid railway power substation systems and railway smart grid perspective. Gopstein reviewed the historical grid and the changing grid of today and claimed that EES can support a more flexible grid realization with improved system reliability and resilience [5]. In addition, the technical characteristics, utility-scale grid applications with their impacts, and the deployment of EES were discussed [5]. Whittingham discussed the importance of EES in the key application areas (including electronic devices, transportation, and utility grids), and predicted the EES capabilities in conjunction with the smart grid [6]. Luo et al. provided a comprehensive study of the recent development of EES technologies in both academic communities and industrial sectors [2]. The study was carried out based on the relevant technical and economic data, and the further discussion on the EES power system applications with their decision-making factors was also given [2]. From another point of view, some other articles reviewed the Grid Code technical requirements with different focuses, e.g., the international regulations and the current practices regarding the verification and certification of the electrical performance in renewable generation systems for grid connection [7], the requirements to generate assets with the influence of weakness and isolation of a power grid on the interconnection conditions [8], and the Fault Ride Through review concerning on photovoltaic systems to power networks [9]. From the above, the review work of Grid Codes in relation to the EES applications which are for supporting/achieving the power grid operating regulations’ realization is quite necessary. This paper provides a comprehensive study mainly on the voltage and frequency Grid Code specifications, for investigating the relevant EES applications aiming to meet the grid operation regulations and also for guiding the corresponding technology development. This paper begins with an overview of the Grid Codes through a detailed study of Great Britain (GB) Grid Code with a comparison of many other countries’ grid operation regulations. Then, a technical analysis is performed to identify whether the EES technologies can meet the Grid Code requirements. This paper also evaluates the different EES technology application potentials for supporting the Grid Code realization, especially on frequency and voltage control. Finally, some recommendations are made for technology development, in terms of supporting the grid operation and helping the Grid Code evolution. 2. Overview of Grid Codes The Grid Code differs considerably from one country to another, because they are directly related to the nature of generation characteristics and network operation requirement. For instance, the frequency response requirement is normally more stringent in a relatively isolated (i.e., weakly interconnected) system, such as the grid in Great Britain and Ireland, compared with a large and strongly interconnected system, such as the French transmission system in Continental Europe. Grid Energies 2018, 11, 1070 3 of 26 Code requirements were initially developed based on the conventional fossil-fuelled power plant operation characteristics and since then have been tailored to allow more different generation types connecting to the power network, for example, wind power generation. For managing the specific national grid systems and dealing with different situations including emergencies, TSOs set their own Grid Code specifications individually. An overview of Grid Code requirements in different countries is presented in this section, mainly in the aspects of voltage levels, normal/critical frequencies with intervals and requirements to generating units. It should be noted that the national regulatory frameworks are subject to continuous changes and revisions. 2.1. Voltage Adopted by National Electricity Transmission The GB Grid Code is applied to power networks with transmission voltage levels of 400, 275 and 132 kV (32 kV for Scotland) [1,7,10]. The national high-voltage transmission system is owned and maintained by three companies: National Grid (owns more than 14,000 circuit km of 400 kV and 275 kV overhead lines and cables), Scottish and Southern Energy (about 5000 circuit km of 275 kV and 132 kV overhead lines and cables) and Scottish Power (about 4000 circuit km of 400, 275 and 132 kV overhead lines and cables) [1]. Table1 summarizes the operating